The prior art deals with several sensorless control algorithms all of which are based on the knowledge of voltages and currents of the motor phases (direct torque control or direct field oriented control). By measuring the voltages and currents of the motor phases and by knowing the mathematical model of the motor, it is possible, with an integral calculus operation, to determine estimates for the stator flux components (see Formula 4,5,6 and 7) and the rotor flux components.
By a real time calculation of the magnitude and of the spatial orientation of said flux components, it is possible to apply the principles of the field oriented control. Moreover, simplified algorithms are well-known (sometimes called quasistatic), having less temporal variability than the field oriented control (e.g. constant V/f and slip control) that use the magnitude of the flux components only to apply a coarse speed regulation of an induction motor.
All of these methods (direct field oriented control and quasistatic) have a big drawback: the accuracy of the estimation of the flux components is very poor at low frequencies.
The reason is simple: for a fixed flux magnitude, the stator EMF is proportional to the applied frequency whereas the current in the motor does not depend on the frequency but only on the load torque. Therefore, drops of the stray parameters of the motor (stator resistances but also leakage inductances) are proportional to the current resulting in a strong influence when compared with the stator EMF at low frequencies; as the frequency increases this influence becomes more and more negligible.
In fact, the estimation of the stator flux is done by subtracting the drops of the stator resistances from the phase voltages and then integrating (see Formulas 6,7,4 and 5).
As above justified, it is at low frequency that the estimation of the flux components may be inadequate, because of the poor accuracy due to the error between the actual stator resistance and the one used in the calculation.
It is evident that the stator resistance used in the calculation has an intrinsic error: it is just enough to think at the thermal drift of the actual stator resistance, at the differences between the stator resistance of two phases, or at the drift and differences in the contact resistance of the motor and inverter connections.
The consequence is that the performance at low frequencies, at best, will be degraded but sometimes it can be totally compromised due to the error in the stator resistance estimation.
In the above considerations the drops in the leakage inductances have been deliberately ignored. We did that because these drops are not involved in the calculation of the motion torque. It will be shown next that the highlight of a control, without the encoder, is the accurate determination of the motion torque. We will see also that the torque determination makes use of the stator flux and motor current components.
To engross the above explanation it is necessary to refer to the following Formula 3 that is the general expression of the motion torque in an induction motor. The flux and current components in this formula are referred to according to an equivalent two phase model of the three phase motor. It is well known that it is possible to switch from the three phase model to the two phase model with the application of the Clarke transformations (Formula 1 and 2) below.
                                          [                                                                                φ                    ⁢                                                                                  ⁢                    qs                                                                                                                    φ                    ⁢                                                                                  ⁢                    ds                                                                        ]                    =                                                    D                ·                                  [                                                                                                              φ                          ⁢                                                                                                          ⁢                          as                                                                                                                                                              φ                          ⁢                                                                                                          ⁢                          bs                                                                                                      ]                                            ⁢                                                          ⁢              with              ⁢                                                          ⁢              D                        =                          [                                                                                          1                                              3                                                                                                                        2                                              3                                                                                                                                  1                                                        0                                                              ]                                      ⁢                                  ⁢                  and          ⁢                                          ⁢          vice          ⁢                                          ⁢          versa                                    Formula        ⁢                                  ⁢        1                                          [                                                                      φ                  ⁢                                                                          ⁢                  as                                                                                                      φ                  ⁢                                                                          ⁢                  bs                                                              ]                =                                                            D                                  -                  1                                            ⁡                              [                                                                                                    φ                        ⁢                                                                                                  ⁢                        qs                                                                                                                                                φ                        ⁢                                                                                                  ⁢                        ds                                                                                            ]                                      ⁢                                                  ⁢            with            ⁢                                                  ⁢                          D                              -                1                                              =                      [                                                            0                                                  1                                                                                                                        3                                        2                                                                                        -                                          1                      2                                                                                            ]                                              Formula        ⁢                                  ⁢        2                                Tm        =                              3            2                    ⁢                      p            ⁡                          (                                                iqs                  ⁢                                                                          ⁢                  φ                  ⁢                                                                          ⁢                  ds                                -                                  ids                  ⁢                                                                          ⁢                  φ                  ⁢                                                                          ⁢                  qs                                            )                                                          Formual        ⁢                                  ⁢        3            With:                Tm: motion torque.        φqs, φds: stator flux components in the two phases equivalent model.        φas, φbs: stator flux components in the the a and b phases of the three phase model.        iqs,ids: stator current components in the equivalent two phase model. They are obtained from the application of the Clarke transformation for the ias, ibs pair of the original three phase motor.        p: poles pair number        
Formula 3 states that, by knowing the stator flux and current components it is possible to perform the real time calculation of the motion torque in magnitude and sign: the sign informs whether the torque is in the direction of the applied frequency or in the opposite one.
This is all we need to get a continuous monitoring of the state of the motor control. By knowing an accurate estimation of the motion torque and its direction, it is possible to decide without ambiguity whether the control works consistently with the state of the command or not. The well known low level control algorithm, consisting of a distinct modulation of the current component giving the torque and of the current component giving the flux, is a secondary point with respect to the possibility to apply a continuous supervision of the control.
In fact, we expect that, when the magnitude of the current in the motor is high, the torque must be high too and we expect its sign to be positive (motoring torque) when the actual frequency is lower than the commanded frequency; we expect the sign of the torque to be negative (braking torque) when the actual frequency is higher than the commanded frequency. Obviously, this holds for the case that the control tries correctly to reach the commanded frequency. Then, it can happen that the control is not able to reach the commanded frequency because of too much torque required (e.g. the vehicle is on a high grade with a load).
In this case, we expect that the controller works at the maximum allowed current with the maximum magnetic flux with the maximum torque magnitude and the proper torque sign being positive when the actual frequency is lower than the commanded frequency, and being negative when the actual frequency is higher than the commanded frequency.
But, above all, this continuous monitoring of the magnitude and sign of the motion torque allows to recognize when the working point is falling in the unwanted unstable area of the motor characteristic (too high slip and torque collapsed area): in fact this condition is represented by a huge current in the motor together with a very low supplied motion torque magnitude. If we are able to detect when the working point lies in the unwanted unstable area, it is quite sure we can avoid to fall in that trap.
So, the accurate measurement of the actual motion torque represents the highlight of the control, because it becomes possible to recognize when the working point of the motor is not consistent with the command or when the working point falls in the unstable area of the motor characteristic allowing to apply a real time countermeasure or a corrective action. These countermeasures will operate on the well known low level control algorithm and consist of a proper modulation of the flux, of the armature current and of the frequency, in order to control the motor again properly.
The above is supplied to make evident the high importance of a correct torque estimation. Besides, for a correct modulation of the flux in the low level algorithm, the high importance of a correct flux estimation is also obvious. The conclusion is that the control without the encoder at the motor shaft, may work properly, if the calculation of the Formula 3 (and also the flux estimation) gives accurate results.
It is one goal of the present invention to provide a measurement of the magnetic flux components instead of a bare estimate: then the torque and flux calculations will have the suited level of accuracy for a consistent applying of the Formula 3 and for an accurate flux measurement. This goal, together with other ones, are reached by the method for a direct measurement of the electromotive forces in an induction motor (to be used in a direct control of the motor self), and by the motor that makes possible this direct measurement. Method and motor are characterized as explained in the annexed claims.